Substantial evidence suggests that serotonin systems can modulate anxiety behaviors, and the most common pharmacological treatments for anxiety disorders are drugs that target serotonin systems. Exercise has also been shown to be therapeutic in the treatment of anxiety, and the effects of exercise may be mediated through changes in serotonin functioning. The proposed studies are designed to investigate the modulation of a brain region critical for anxiety behaviors, the bed nucleus of the stria terminalis (BNST), by serotonergic drugs, and whether the effects of exercise on anxiety are mediated by serotonin changes in the BNST. Metachlorophenylpiperazine (mCPP), which is an agonist at 5-HT2B and 5-HT2C receptors (with some affinity at 5-HT2A receptors), increases signs of anxiety in both humans and animals. The BNST has been implicated in mediating anxiety in a number of paradigms in rodents and primates. Therefore, the BNST might an important brain region mediating the effects of serotonergic modulation on anxiety. Single BNST neurons can respond to 5-HT with excitation and/or inhibition, and mCPP injected into the BNST is anxiogenic likely because this drug selectively activates the excitatory response on these neurons, which is mediated, in part, by the 5-HT2 family of receptors. Consistent with reports in humans, we have shown that voluntary exercise is anxiolytic in many anxiety models in animals. Importantly, voluntary exercise reduces the increases in anxiety observed after mCPP. The studies in this proposal are designed to investigate the mechanisms by which exercise protects organisms against the anxiogenic effects of mCPP, using behavioral, pharmacological, and electrophysiological methods. In Aim 1 of this proposal, we will investigate whether the effects of intra-BNST mCPP are mediated by 5-HT2A or 5-HT2C receptors by determining the dose-response curves of mCPP's behavioral effects in the presence of 5-HT receptor subtype selective antagonists, also injected into the BNST. We will also then investigate the effects of voluntary exercise on the dose-response curve of intra-BNST mCPP. In Aim 2, we will use whole-cell patch clamp techniques on BNST slices in vitro, to determine whether voluntary exercise shifts the population of serotonin responses to the favor inhibition, and whether the same treatment alters the dose-response curve to exogenously applied mCPP. We will use selective antagonists to isolate the effects of mCPP to each 5-HT2 receptor subtype, to determine which subtype(s) is/are altered by voluntary exercise. It is expected that voluntary exercise will decrease the function of one or more of the subtypes of the 5-HT2 family of receptors. Anxiety disorders affect approximately 40 million American adults and the treatment of these disorders is extremely expensive. The experiments in this proposal are designed to investigate the mechanisms by which exercise, a low-cost alternative to drugs that also promotes other forms of health, reduces anxiety. Understanding these mechanisms will provide critical insight for both the etiology and treatment of anxiety disorders.